net/http: clarify influence of go.mod version on ServeMux behavior

[donmahallem]

Added a note about the importance of the specified go.mod version.

When using go 1.25 with a 1.20 specified in the go.mod file I had non
obvious issues with getting the new path behavior(since 1.22) to work.

I do recognize that https://go.dev/doc/modules/gomod-ref#go-notes specifies
that gowill refuse to use "future" features but as the interface is the same
there are no compiler errors. As it took me quite some time to catch it would be
great to have a note about this as well in the docs.

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